(a) Field of the Invention
The present invention relates to a liquid crystal display, more particularly, to a liquid crystal display having the improved image quality and viewing angle.
(b) Description of the Related Art
Generally, a liquid crystal display includes an upper panel having a common electrode and a plurality of color filters, a lower panel having pluralities of thin film transistors (TFTs) and pixel electrodes, and a liquid crystal layer between the two panels. The pixel electrodes and the common electrode are applied with different electric potentials to generate electric fields which vary the arrangement of liquid crystal molecules to control the light transmittance, thereby displaying images.
The initial arrangement of liquid crystal molecules can be, for example, horizontally twisted or vertically aligned. A vertically aligned structure can produce a relatively high contrast ratio and a relatively wide viewing angle. In a vertically aligned mode liquid crystal display, longitudinal axes of liquid crystal molecules are aligned perpendicular to the upper and the lower panels in absence of electric field. When electric field is applied, the liquid crystal molecules are tilted with respect to an axis normal to the panels at angles dependent on the strength of the electric field, so that the polarization of light passing through the liquid crystal layer can be changed. The larger the tilt angle of the liquid crystal molecules, the larger the amount of change in polarization. If the polarization axes of a polarizer and an analyzer opposite each other with respect to the liquid crystal layer are aligned to be perpendicular to each other, the light which is linearly polarized after passing through the polarizer experiences no change of the polarization in the liquid crystal layer in absence of electric field, thereby being almost completely intercepted by the analyzer. Accordingly, the screen of the liquid crystal display becomes black. However, when electric field is applied, the polarization of the linearly polarized light changes in the liquid crystal layer. Accordingly, when light reaches the analyzer, the light has a polarization component parallel to the polarization axis of the analyzer, and thus the light out of the analyzer is visible to users. Generally, the higher the strength of the electric field applied to the liquid crystal layer, the larger the magnitude of the polarization component parallel to the polarization axis of the analyzer, and the screen of the liquid crystal display becomes brighter.
Based on the above principle, a liquid crystal display realizes gray scale by adjusting the potential difference between the pixel electrodes and the common electrode. Generally, the gray scale ranges from the first gray representing the darkest state to the sixty-fourth gray representing the brightest state.
In the meantime, the contrast ratio of a liquid crystal display when viewed from its lateral sides is very poor compared with when viewed from the front of the display. The contrast ratio is defined as the luminance of the sixty-fourth gray divided by that of the first gray. In addition, the color reproduction when viewed from the lateral sides is inferior to that of the front view. A liquid crystal display having poor contrast ratio and inferior color reproduction cannot display bright and sharp images.